More than 300 workers were inside the plant in Jilin province when the fire broke out about 6 a.m., Xinhua said.

About 100 of them were able to escape -- and told Xinhua that the gates to the plant were locked.

"Suddenly, the lights inside went out and the plant got quite dark," a worker named Wang Fengya told the news agency.

"When I finally ran out and looked back at the plant, I saw high flames."

Firefighters battled the blaze for six hours before they were able to bring it under control.

A fire official told CNN that an explosion at the plant started the fire -- but the cause of the blast wasn't immediately known.

"The complicated interior structure of the prefabricated house in which the fire broke out and the narrow exits have added difficulties to the rescue work, sources with rescue forces said," the Xinhua report said.